As an optical fiber which forms an optical transmission path, there are an SMF which has small transmission loss and is suitable for long-distance transmission and an MMF which has comparatively large transmission loss but is inexpensive and suitable for short-distance transmission.
An optical fiber has a coaxial structure in which a clad portion having a smaller refractive index surrounds the periphery of a core portion. In the optical fiber, light which enters the core portion is reflected (refracted) from the boundary portion between the core portion and the clad portion and propagates in the length direction. An SMF has a minute (in general, 9.2 μm) core such that there is a single propagation mode (passage) when passing through the core portion, and an MMF has a thick core (in general, 50 μm, 62.5 μm) such that there are a plurality of propagation modes.
As shown in FIG. 5, an optical pulse test apparatus which tests a transmission path formed by an optical fiber provides an optical pulse Pin emitted from a light source 11 to a connector 13 through an optical coupler 12. The optical pulse Pin enters a test-target optical transmission path 1 connected to the connector 13, and return light (backward-scattered light or Fresnel reflected light) Pr from the optical transmission path 1 enters an optical receiver 14 through the connector 13 and the optical coupler 12. Data regarding the intensity of return light Pr received by the optical receiver 14 from the entering timing of the optical pulse Pin is continuously acquired for a given time, and the presence/absence of the occurrence of failure in the optical transmission path 1, or the like from acquired data is examined.
As described above, with regard to the optical fiber which forms the optical transmission path 1, an SMF is used for long-distance transmission, and an MMF is used for short-distance transmission. In the optical pulse test apparatus of the related art, however, it is not possible to test an optical transmission path using an SMF and an optical transmission path using an MMF by a single test apparatus due to a difference in the core diameter.
For example, in the case of an optical pulse test apparatus for SMF measurement, a connection portion of the connector 13 to the core of the optical fiber is designed for the core diameter of the SMF. When an MMF having a significantly thick core diameter is connected to the connector 13, an optical pulse enters from a small core diameter to a large core diameter, such that a large loss does not occur. Meanwhile, with regard to return light from the MMF, return light enters from a large core diameter to a small core diameter, such that a considerable loss occurs.
To the contrary, in the case of an optical pulse test apparatus for MMF measurement, a connection portion of the connector 13 to the core of the optical fiber is designed for the core diameter of the MMF. For this reason, if a SMF having a significantly fine core diameter is connected to the connector 13, a considerable loss occurs when an optical pulse enters the SMF.
As a technique for solving this type of problem, in an optical pulse test apparatus for MMF measurement, a technique is known in which outgoing light is excited in a low-order mode, thereby reducing a loss when light enters from the MMF to the SMF. The polarization of outgoing light is changed to prevent fluctuation in the light reception intensity due to the polarization dependency of an optical path conversion element (corresponding to the optical coupler 12) (for example, Patent Document 1).